1. Field of the Invention
The present invention relates to an electric power steering apparatus.
2. Related Background Art
There has been known an electric power steering apparatus for a vehicle in which the rotational output of an electric motor acting as an auxiliary steering torque is reduced to a suitable torque by a gear train and transmitted to the output shaft of a steering mechanism at a suitable speed. When such a steering apparatus is operated, there occurs a problem in which noise is produced between the driving gears and the driven gears of the gear train. Japanese Laid-Open Utility Model Application No. 2-108657 discloses one method of preventing such noise by forming teeth made of resin on a cylindrical metal core portion of at least either one of the driving gear and the driven gear so that noise are reduced upon engagement of the gears.
In a compound gear comprising a metal core portion and resin teeth, the coefficient of linear thermal expansion of resin is .larger than that of metal. When, therefore, the thickness of resin from the addendum circle to the outer surface of the metal core portion is set to be relatively thick, the following problems occur. If the compound gear is used in an atmosphere at a high temperature, the resin portion of the compound gear is expanded remarkably. The backlash of the compound gear is reduced and the torque transmission efficiency of the compound gear and its related portion is lowered. Further, the sliding portions are worn much. When the temperature of the atmosphere around the compound gear whose backlash is determined so as to be used in the atmosphere at a high temperature is changed from a high value to a low value, the resin portion of the compound gear shrinks remarkably. Thus, the backlash of the teeth increases beyond the tolerance, noise of the teeth produced upon engagement becomes high, and delay of power transmission occurs.